1. Field of the Invention
The present invention relates to an encoder to be used in various apparatuses, such as optical apparatuses, which outputs a signal that indicates position of a movable member provided in such apparatuses, the encoder outputting the signal by relative movement of a scale and a sensor with movement of the movable member.
2. Description of the Related Art
Encoders used to detect position or velocity of a movable member are each constituted by a scale which is provided with a periodic pattern and a sensor which outputs a periodic signal that periodically changes depending on the periodic pattern in response to relative movement of the scale and the sensor. The encoders are classified into incremental encoders each detecting a relative position of the scale and the sensor, and absolute encoders each detecting an absolute position of the scale or the sensor.
Japanese Patent Laid-Open No. 2009-198318 discloses a photoelectric absolute encoder formed such that its scale includes multiple tracks thereon each provided with a periodic pattern whose shape or pitch periodically changes in a position detection direction (or a length measurement direction). This encoder can detect an absolute position of the scale with high accuracy by using a combination of signals which are acquired by detection of the periodic patterns provided in the multiple tracks and whose periods are different from each other.
The encoder disclosed in Japanese Patent Laid-Open No. 2009-198318, however, detects the periodic patterns provided in the multiple tracks by using multiple light-receiving element arrays each provided to the sensor for each of the periodic patterns. With this configuration, accuracy in acquiring the periodic signals may be decreased due to a displacement of mounting positions of a light source and the light-receiving element arrays in a height direction (in which light transmitted through or reflected by the periodic patterns travels toward the sensor) and due to an error of an image magnification caused by a relative inclination between the scale and the sensor. This consequently makes it impossible for the encoder to perform highly accurate position detection.